This invention relates to a system and method of decreasing nitrogen oxides ("NO.sub.x ") emissions from a fluidized bed reactor. More particularly, this invention relates to the selective injection of a reactant into the reactor for reducing NO.sub.x levels in gaseous products of combustion in the reactor.
Fluidized bed combustion systems are well known and include a furnace section in which an oxygen-containing gas such as air is passed through a bed of particulate materials, including nitrogen-containing, carbonaceous fuel material, such as coal. Sorbent particles, such as limestone, lime, or dolomite may be added for the capture of oxides of sulfur generated during combustion. The oxygen-containing gas fluidizes the particulate materials in the furnace section and promotes the combustion of the particulate fuel material at a relatively low temperature. These types of combustion systems are often used in steam generators in which a cooling fluid, such as water, is passed through a fluid flow circuit in a heat exchange relationship to the fluidized bed reactor to generate steam and to permit high combustion efficiency, fuel flexibility, high sulfur adsorbtion, and relatively low NO.sub.x emissions.
A typical fluidized bed reactor utilized in the generation of steam is commonly referred to as a "bubbling" fluidized bed in which the fluidized particulate materials form a bed having a relatively high density and a well-defined or discrete upper surface. A more commonly used fluidized bed reactor is referred to as a "circulating" fluidized bed in which the fluidized particulate materials form a lower dense bed having a density below that of a typical bubbling fluidized bed and in which the primary gas has a fluidizing velocity which is equal to or greater than that of a bubbling bed. The primary gas passing through the lower dense bed entrains a substantial amount of fine particulate materials to form an upper dispersed bed of particulate materials, often to the extent that the primary gas is substantially saturated with the particulate materials in the dispersed bed.
It is generally considered desirable to operate these circulating fluidized beds using relatively high internal and external solids recycling so that they are insensitive to fuel heat release patterns, thus minimizing temperature variations and stabilizing the sulfur emissions at a low level. The high external solids recycling is achieved by disposing a separator, such as a cyclone separator, at the furnace section outlet to receive the flue gases, and the particulate materials entrained thereby, from the dispersed bed of the furnace section. The entrained particulate materials are separated from the flue gases in the separator, and the cleaned flue gases are passed to a heat recovery section while the separated particulate materials are recycled back to the furnace section. This recycling improves the efficiency of the separator, and the increased residence times of the fuel and sorbent particles result in more efficient use of the fuel and sorbent particles and, therefore, reduced consumption of the same.
Bubbling and circulating fluidized bed reactors also offer advantages in pollution control. For example, the emissions of NO.sub.x from fluidized bed reactors are relatively low compared to emissions from other conventional systems such as gas-fired systems and coal-fired power plants. To obtain even lower NO.sub.x emission levels, selective non-catalytic reduction ("SNCR") methods and selective catalytic reduction methods ("SCR") are employed. In SNCR methods, a reactant such as urea or ammonia, is injected into the reactor to react with the NO.sub.x, forming N.sub.2 and H.sub.2 O. The reactant is typically injected through numerous ports at various locations across the reactor including the furnace section, the separator, and the duct connecting the furnace section and separator. SNCR methods thereby allow even lower NO.sub.x emission levels to be obtained.
However, SNCR methods are not without problems. For example, inefficient utilization of the added reactant often prevents the SNCR methods from obtaining the desired degree of decrease in NO.sub.x levels. For more efficient usage of the reactant, it is desirable to have a high residence time of the reactant in the system, a high degree of mixing of the reactant with the NO.sub.x -containing flue gases, and a low degree of mixing of the reactant with the particulate materials circulating in the system. Present systems often suffer from inefficient use of the reactant. For example, systems which inject the reactant into the furnace section and systems which inject the reactant into various locations across the duct may suffer from too much mixing of the reactant with the particulate materials and insufficient mixing of the reactant with the NO.sub.x -containing flue gases. Similarly, systems which inject the reactant into the separator may suffer from insufficient residence time and from insufficient mixing of the reactant with the NO.sub.x -containing flue gases.
Inefficient utilization of the reactant results in excessive use of the reactant which adds to the cost of the SNCR method. Additionally, adding excessive amounts of reactant can generate new pollution problems.